Recent efforts to improve the electrical performance of both silicon and III-V compound semiconductor solar cells are well-known and well documented. Gallium arsenide-gallium aluminum arsenide solar cells have recently been described by J. d'Beau in an article entitled "From Photons to Kilowatts: Can Solar Technology Deliver?" Electronics, Nov. 11, 1976, on pages 86-90. See also J. M. Woodall et al., Applied Physics Letters, 30, 492 (1977). Efforts in this solar cell technology include, among other things, attempts to increase the solar cell conversion efficiency, and one approach to this problem has been the utilization of stacked or tandem connected semiconductor wafers in a structure responsive to different wavelength ranges of sunlight. Ideally, solar cells fabricated by stacking separate wafers will have PN junctions with different bandgap energies which are responsive to different wavelength ranges in the electromagnetic wavelength spectrum. These PN junctions will generate a combined power output which is greater than the individual power outputs from the individual PN junctions. One problem with this first approach to increasing solar cell efficiency is that two semiconductor wafers are approximately twice as heavy and twice as expensive as one and therefore make this stacked wafer approach unacceptable for certain light weight spacecraft applications. Alternately, if this first approach is used in terrestrial applications in combination with high concentration ratio optical systems, it also presents severe cell cooling problems.
A second approach to the problem of increasing the efficiency and output power of solar cells is to epitaxially grow multiple layers of different III-V semiconductor compound materials vertically upward from one surface of a suitable substrate material. The difficulty with this latter approach resides in the very high degree of process control required for growing multiple III-V compound epitaxial semiconductor layers of different materials atop a single substrate starting material. Not only is this latter process approach inherently disadvantageous from the standpoint of achievable process yields and fabrication costs; it also suffers from the limitation that the multiple epitaxial layers grown atop a single substrate surface are inherently inaccessible to contacts from both sides of each individual PN junction.